This aspect's most effective spell against mobs. Herd together as large group in as open an area as possible and cast.
In the PC version, right-clicking close to the character will result in a wider angle of the tempest (can be >180°) but lower damage on initial hit. Right-clicking farther away from the character will result in a tempest of a smaller angle that will do more damage on initial hit. In the console version, the spread of the tempest is determined by the distance between the player and the enemy targeted.
As the damage over time is larger than the initial damage, a way to quickly kill mobs can be to cast and move on, letting the DOT finish them off. This can be a very fast way to gain experience. The disadvantage of this method is that you leave behind treasure when the mobs die.
Even if playing an elf that is not using any pyromancer skills, Blazing Tempest may be a way to quickly kill lots of enemies at the beginning (first 5-10 levels) and quickly gain experience.